License Plate Capture Cameras

At the front end of any Automatic Number Plate Recognition (ANPR) system is the imaging hardware, which captures the images of the license plates. The initial image capture forms a critically important part of the ANPR system, which - in accordance to the Garbage In, Garbage Out principle of computing - often determines the overall performance.

License plate capture is typically performed by specialized cameras, which are designed specifically for the task. Factors which pose difficulty for license plate imaging cameras include speed of the recorded vehicles, varying ambient lighting conditions, headlight glare and harsh environmental conditions. Most dedicated license plate capture cameras incorporate infrared illumination to solve both lighting and plate reflectivity problems.

To avoid blurring, it is ideal to have the shutter speed of a dedicated camera set to one-thousandth of a second, because as the car moves, slower shutter speeds could result in an image too blurry to read using Optical Character Recognition (OCR) software, especially if the camera is higher than the vehicle. In either slow-moving traffic - or when the camera is at a lower level and the vehicle is at an angle approaching the camera - the shutter's speed is not as important: shutter speeds of 1/500th of a second are designed for traffic moving up to 40 miles-per-hour (mph) - or 64 kilometers-per-hour (km/h) - and at 1/250th of a second, can handle traffic moving up to 5 mph (8 km/h). And keep in mind license plate capture cameras can now produce usable images from vehicles traveling at 120 mph (190 km/h).

So, to maximize the chances of effective license plate capture, installers should carefully consider the positioning of the camera relative to the target capture area. Exceeding threshold angles of incidence between camera lens and license plate greatly reduces the probability of obtaining usable images, due to distortion. However, manufacturers have thankfully developed tools to eliminate errors from the physical installation of license plate capture cameras.